May 2000 1/2
AN923
APPLICATION NOTE
TIMEKEEPER R o lling into the Year 2000 (21st Century, Y2K)
Th e me mb e rs o f the TI MEKEEPER famil y, from STM icroelectronics , a re listed in Table 1. Each conta ins,
in a single-package footprint, non-volatile RAM (NVRAM) and a real time clock (RTC), offering dedicated
time-keeping and alarm functions.
CENTURY ROLLOVER
The date and time information is distributed over several internal registers, with two BCD digits in each
register. Fabricated in ST’s SRAM technology, the individual registers can be accessed by the user like a
conventional bank of SRAM (read accesses to read the time, write accesses to set a new time).
The year is divided into tw o fields: the Centu ry field, a nd t he Years field (each i s 8 bits wide, holding two
BCD digits). TIMEKEEPER devices that were designed in 1997, and after, have a register for each of
these fields; those that were designed before 1997, though, do not have the Century register.
At the turn of the century, all TIMEKEEPERS will increment the Years register from 99 to 00 (from
1001,1001 to 0000,0000). Those that have a Century register, will, at the same moment, increment it from
19 to 20 (from 0001,1001 to 0010,0 000).
The Century register, where present, is positioned 14 address locations before the most significant byte
address, using the device’s 16-byte register map definition. For a 32K x 8 device, for example, the Century
register is at addre ss location 7FF1h.
Fo r TIMEKEEPER de vices that do no t h a ve a C e nt u ry r e g ister, but which utilize the 16-byte register map
definition, it is recommended that the user sets up the same location (location 7FF1h for a 32K x 8 device,
for example) for storing the Century information. This byte, of course, is not updated automatically by the
clock, but has to be upd ated manua lly by the user’s software. It is recom m ended that the us er stores the
BCD value 19 (0001,1001) in the location now, and on 1st January 2000, at 12:00:01 AM, updates it to
the BCD va lue 20 (0010,0000).
TIM EKEEPER device s that do not ha ve a Century regi ster, and that do not utilize the 16-byt e register map
definition, can still adopt the same methodology. Any address location that is convenient to the application
can be used for storing the Century information.
Note: All TIMEKEEPERS automatically allow for 31, 30, 29 and 28 day months (correctly adjusting for
leap-years until the year 2100).
Table 1. Family of TIMEKEEPER Devices
M41T56 Low cost RTC that is compatible with Serial I2C bus applications
M48T86 RTC primarily utilized for its timekeeping functions with little additional memory
M48T559 RTC with 64 Kbit memory that supports Address, Data Multiplexed I/O pins
M48T02, M48T12, M48T08, M48T18,
M48T58, M48T59, M48T35, M48T37 RTCs with 16 Kbit to 256 Kbit of additional asynchronous SRAM non-volatile
memory
M48T201 TIMEKEEPER SUPERVISOR
AN923 - APPLICATION NOTE
2/2
If you have any questions or suggestions concerning the matters raised in this document, please send
them to the following electronic mail addresses:
apps.nvram@st.com
(for application support)
ask.memory@st.com
(for general enquiries)
Please remember to include your name, compa ny , location, telephone num ber and fax number.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No lic ense is granted
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to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as cri tica l c om ponent s in l i fe su pport devices or system s wit hout e xpress writ ten ap proval of STM i croelec tronics.
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